Some metals are recovered from their mined ores by the use of aqueous solutions to leach or dissolve the desired metal from such ore. The aqueous solution used for a given metal contains some chemical agent that converts the metal to a soluble form. For instance, gold and silver may be recovered from a gold-containing and silver-containing ores by leaching such ores with an aqueous solution of cyanide (routinely but not necessarily sodium cyanide) to form gold cyanide and silver cyanide, or complexes thereof. Such species are soluble in water or at least in the aqueous leaching solution at the concentrations sufficient for pregnant liquors. Gold and silver in mined ores are typically in their elemental states. As another example, base metals such as copper (typically as copper oxide) and nickel, which generally are not found in their elemental states in ores, are leached from ore with dilute sulfuric acid solution, which dissolves the metal as the sulfate (SO.sub.4.sup.-) form.
After leaching, the desired metal is recovered from the leaching solution, which is referred to as the "pregnant liquor" when it contains the solubilized metal. For instance, an aqueous solution that contains gold cyanide may be directly treated with zinc dust whereby zinc is oxidized to zinc cyanide, and the gold of the gold cyanide is reduced to elemental gold. Other methods are used for other metals.
At the time the pregnant liquor is processed to release the metal, such liquor has of course been separated from the undesirable, and generally undissolved, ore constituents, known as "mud", "mud refuse", or "gangue", or as "tailings" when spent in the sense of being substantially depleted of the desired metal value.
One method of leaching generally is called "heap leaching". In a heap leaching method, the ore is placed in an immense stack or heap, which heap is then leached by percolating the leaching solutions from the top of the heap down, and collecting the effluent (the pregnant solution from which the desired metal is recovered) from the bottom. Heap leaching is a commercially viable method of extracting the desired metal from low grade ores.
There exist large deposits of refractory ores that are not amenable to conventional leaching. The metal, such as gold, may be physically entrapped in a matrix of sulfide metals, such as iron pyrite and arsenopyrite. The matrix is not penetrated by cyanide solution and thus a cyanide solution alone cannot extract and dissolve the gold from such ore. The metal may also itself be chemically bound within a sulfide matrix, such as when base metals are in a sulfide form, and as such cannot be leached with sulfuric acid.
One method being explored commercially is the bacterial or biological oxidation or digestion of the sulfide material to sulfates. Such "bio oxidation" will break down a sulfide matrix, freeing the elemental gold and thus allowing contact of the gold with the dissolving solution. Such bio oxidation will also oxidize base metals sulfide(s), which then can be solubilized in sulfuric acid.
When bacterial digestion of ore is used in a heap leaching method, the ore, after heaping, is treated with the bio solution, which is applied to the top of the heap by sprinkling and allowed to percolate or seep down through the heap and therein attack the sulfide matrix. The treatment with the bio solution is continued until the practical limits of sulfide matrix oxidation are met. Then the leaching with the metal solubilizing solution may begin. This mining process is referred to as bacterial heap leaching or bacteria-assisted heap leaching.
The conditions of the bio oxidation step may be, however, extremely different from those of the metal leaching. The bio oxidation is conducted under extremely acidic conditions, while the cyanide solution leaching of gold or silver is, for safety reasons, conducted using extremely alkaline conditions. The bio oxidation releases ferric ions, and thus the bio solution as used commercially has a very high iron content, in contrast to precious metal leaching solutions, which have extremely low iron contents because iron hydrous oxide is removed by precipitiation in the high pH environments thereof. Some base metal leaching solutions, such as copper, do have high ferric ion concentrations, for instance of the order of 5 grams of ferric ion per liter of solution, but the aqueous solution used to apply the agglomeration aid would not normally have a high iron concentration.
The ore heap must remain permeable to the treating solutions, and to the air, during both the bacterial oxidation and the leaching of the metal. Channelling, blinding, ponding, slumping and other consequences will occur if heap permeability is lost, or diminished, or lost in portions of the heap, and such consequences will at minimum decrease the efficiency of the bacterial heap leaching process.
U.S. Pat. No. 4,875,935, Gross et al., October 1989, discloses a method for extracting copper by heap leaching which includes agglomerating the fines prior to heap formation using as the agglomerating agent an anionic polyacrylamide. Such polymer contains at least 5 mole percent of carboxylate or sulfonate groups, and has a molecular weight of at least 100,000. U.S. Pat. No. 5,100,631, Gross, March 1992, discloses a method for extracting gold and silver by heap leaching which includes agglomerating the fines prior to heap formation using as the agglomerating agent a water-soluble vinyl polymer having a molecular weight of at least 500,000, used alone or in conjunction with inorganic co-agglomerating agents such as cement. Such polymers include acrylamide copolymers having from 5 to 95 wt. percent of acrylamide and the comonomers can be sulfonate-containing and carboxylate-containing materials. U.S. Pat. No. 5,077,021 and U.S. Pat. No. 5,077,022, both Polizzotti, Dec. 31, 1991, disclose the use of a high molecular weight anionic polyacrylamide, a copolymer of acrylic acid and acrylamide, alone, or together with cement, to agglomerate ore prior to heap leaching with cyanide solution. U.S. Pat. No. 4,961,777, Perez et al., Oct. 9, 1990, discloses for refractory ores the incorporation of hypochlorites into the ore agglomerates as they are formed, whereby the hypochlorites act on the refractory components, such as sulfidic matter, as an oxidative pretreatment, in a heap leaching method. The agglomeration aid employed is Portland and/or gypsum cement.
It is an object of the present invention to provide for a bacterial-assisted heap leach process an agglomerating aid treatment that forms a strong agglomerate of the heaped ore, reduces channeling of the percolated solution(s), reduces blinding and ponding (wherein the heap becomes so impermeable to the solution(s) to be percolated that such solution(s) collects on the top of the heap), and provides enduring agglomeration despite the extreme conditions and requirements encountered in a bacterial heap leaching process. These and other objects of the present invention are described in more detail below.